Due to the increasingly number of high rise buildings, the greater sophistication of building designs, etc., fire resistant design codes were revised as a major project of the Japan Ministry of Construction. In March 1987, a new fire resistant design code was enacted. Under this, the old restriction relating to fire resistant coverings requiring the temperature of the steel material at the time of a fire be kept to 350° C. or less was lifted and it was allowed to select a suitable fire resistant covering method in accordance with the high temperature strength of the steel material and the actual load of the building. That is, when possible to secure the design high temperature strength at 600° C., it became possible to eliminate the fire resistant covering.
The 600° C. high temperature strength of a steel material, like the ordinary temperature reinforcing mechanism, is improved by (1) increased fineness of ferrite crystal grain size, (2) solution strengthening by alloy elements, (3) dispersion strengthening by hard phases, and (4) precipitation strengthening by fine precipitates, mainly precipitation strengthening.
Conventional fire resistant steel mainly raises the high temperature softening resistance by precipitation strengthening by carbides of Mo. However, Mo is an expensive element. When the amount added is large, the economy is detracted from, so suppression of the amount of addition is necessary. No addition of Mo is preferable. Furthermore, if the amount of addition of Mo becomes excessive, reheat embrittlement due to precipitation of carbides is feared.
To deal with this problem, fire resistant steel complexly adding Nb, B, and Ti and improving the high temperature strength has been proposed (for example, see Japanese Patent Publication (A) No. 4-350127, Japanese Patent Publication (A) No. 11-302770, and Japanese Patent Publication (A) No. 2000-248335).
However, these do not consider the suppression of the coarsening of the precipitates at the weld heat affected zone (HAZ) at the time of welding. A drop in HAZ toughness is feared.
To deal with such a drop in HAZ toughness, a steel material with the effect of suppression of grain growth by Ti-based oxides and using the intra-granular ferrite nucleation with this as nuclei for growth so as to prevent the coarsening of the crystal grain size at the HAZ has been proposed (for example, see Japanese Patent Publication (A) No. 4-362156).
Furthermore, a method of production of H-section steel utilizing intra-granular ferrite nucleation by Ti-based oxides to make the microstructure even has also been proposed (for example, see Japanese Patent Publication (A) No. 2002-212632).
However, with thick steel plate, shaped steel, and the like, a large heat input is used for welding. The vicinity of the weld zone is heated to a high temperature, so in particular when a HAZ which has been heated once to a high temperature by welding is again heated, the problem arises of embrittlement due to precipitation of carbides and nitrides. The steel materials proposed in these prior patent literature did not consider such HAZ high temperature embrittlement (below, called “reheat embrittlement”).
Further, for extremely thick H-section steel used mainly as columns for high rise buildings as well, along with the increase in thickness and size, the production process becomes lower in reduction rate and lower in cooling speeds, so compared with thin-gauge steel material, sufficient working heat treatment becomes more difficult. Therefore, in the prior art, to secure strength, alloy elements had to be added in large amounts. In that case, a drop in toughness, drop in weldability, and other concomitant problems arose.